Salt Stress Re-Routes Terpenoid and Flavonoid Metabolism in Peppermint (Mentha × Piperita L.)

Salt stress is a major abiotic factor limiting crop productivity. This study investigated the physiological and biochemical responses of peppermint (Mentha × piperita L.) to increasing NaCl concentrations (25, 50, and 100 mM). Terpenoids and flavonoids were analysed by GC–MS and HPLC-ESI-MS/MS, respectively, alongside Real-Time PCR for key biosynthetic genes (menthol biogenesis/Chalcone isomerase) and chlorophyll fluorescence measurements (OJIP, quantum yield, NPQ and PAM). Total terpenoid production decreased significantly (41% at 25 mM, 75% at 100 mM NaCl). Key compounds like menthone (365–71 g/kg), menthol (35–19 g/kg), and 1,8-cineole (58–19 g/kg) were reduced, although pulegone and menthofuran initially increased at moderate salt levels (25 mM) before declining at 100 mM. Mechanistically, this suppression was due to a strategic metabolic bottleneck: upstream terpenoid biosynthetic genes (DXS, GDPS, LS, and L3OH) were upregulated, while terminal-step genes (like menthone/menthol reductase) were progressively downregulated. This metabolic shift correlated strongly with severe photosynthetic impairment, evidenced by reduced quantum yield (0.82–0.61) and altered chlorophyll quenching (0.45–0.28). This impairment likely reduced the precursor supply for terpenoid synthesis. The plant also exhibited selective flavonoid adjustment. While total phenolic content (12.5–7.8 mg GAE g−1 FW) and regulatory gene expression were reduced, the biosynthesis of specific flavonoid conjugates increased (+1.5-fold at high NaCl). These findings reveal that salt stress in peppermint triggers sophisticated and selective metabolic reallocation, severely impacting commercially valuable terpenoids and overall photosynthetic efficiency.